Arc chute for an electric circuit breaker

ABSTRACT

An arc chute for an electric circuit breaker comprises insulating sidewalls and plates, or fins, of insulating material that extend transversely of the sidewalls and have edges for engaging the arc as it moves into the chute. The sidewalls and edges of the plates are coated with a plasma-arc-sprayed refractory metal-oxide consisting essentially of amorphous fused silica.

United States Patent [191 Frind et a].

[451 May 22,1973

[54] ARC CHUTE FOR AN ELECTRIC CIRCUIT BREAKER Inventors: Gerhard Frind, Schenectady, N.Y.; Richard M. Korte, West Chester; John E. Zlupko, Philadelphia, both of Pa.

General Electric Company, Philadelphia, Pa.

Filed: July 14, 1971 Appl. No.: 162,431

[73] Assignee:

[56] References Cited UNITED STATES PATENTS 2,772,334 l1/l956 Latour 200/144C 2,818,345 12/1957 Vickers et a1. ..200/144 C X 2,911,505 11/1959 Leggs et a1 ..200/l44 C 3,009,041 11/1961 Zlupko ..200/149 R X 3,236,979 2/ 1966 Latour ..200/ 144 C 3,560,685 2/1971 Bailey et a1. ..200/ 144 C FOREIGN PATENTS ORAPPLICATIONS 1,553,672 12/1968 France ..200/144 C 1,007,486 10/ 1965 Great Britain ..200/ 144 C Primary Examiner-Robert S. Macon Attorney-J. Wesley Haubner, William Freedman, Frank L. Neuhauser et al.

[ ABSTRACT An arc chute for an electric circuit breaker comprises insulating sidewalls and plates, or fins, of insulating material that extend transversely of the sidewalls and have edges for engaging the are as it moves into the chute. The sidewalls and edges of the plates are coated with a plasma-arc-sprayed refractory metaloxide consisting essentially of amorphous fused silica.

16 Claims, 6 Drawing Figures ARC CIIUTE FOR AN ELECTRIC CIRCUIT BREAKER BACKGROUND This invention relates to an arc chute for an electric circuit breaker and, more particularly, relates to an arc chute that comprises insulating sidewalls and plates, or tins, of insulating material that extend transversely of the sidewalls and have edges for engaging the are as it moves into the chute.

An insulating material which has been widely used for such sidewalls and transverse plates is a refractory material made by reacting concentrated orthophosphoric acid and chrysotile asbestos, as is disclosed for example in US. Pat. Nos. 2,366,485-Brink et a1. and 2,704,381-Nelson. A preferred form of this composition includes, in addition to the asbestos and phosphoric acid, a suitable filler, such as zircon, in an amount of about 60 per cent by weight of the composition. An advantage of this composition is that it is capable of being hot molded and subsequently machined into intricate forms of good mechanical strength suitable for constituting a major portion of the arc chute, for example, for constituting one sidewall and half of the transverse plates of the arc chute. While this material has functioned exceptionally well as an arc chute material, there is an upper limit as to how much current a given size arc chute of this material can consistently interrupt.

SUMMARY An object of our invention is to utilize in the arc chute an insulating material that renders the arc chute capable of consistently interrupting substantially greater amounts of current than can an arc chute of corresponding size made of the zircon-filled phosphoasbestos material referred to hereinabove.

Another object is to substantially increase the resistance of the arc chute material to are erosion as compared to the zircon-filled phospho-asbestos material. By increasing this resistance to are erosion, we can prolong the effective life of the arc chute.

Another object is to achieve the improvements of the two preceding objects without relinquishing the recognized advantages of phospho-asbestos material, such as hot molding capability, machinability, and good mechanical strength.

In carrying out our invention in one form, we form the sidewalls and the transverse plates of the arc chute of a refractory insulating material such as, for example, phospho-asbestos material containing a suitable filler. The sidewalls and the edges of the plates are coated with a plasma-arc-sprayed refractory metal-oxide consisting essentially of amorphous fused silica.

BRIEF DESCRIPTION OF DRAWINGS For a better understanding of the invention, reference may be had to the following description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a side elevational view partly in section showing an electric circuit breaker embodying one form of the invention. One sidewall of the arc chute has been removed to show the interior of the arc chute.

FIG. 2 is a cross sectional view taken along the line 2-2 of FIG. 1, assuming the arc chute is assembled.

FIG. 2a is an enlarged sectional view along the line 2a-2a of FIG. 1.

FIG. 3 is an end view of a portion of the arc chute of FIG. 1.

FIG. 4 schematically illustrates a step in applying the fused silica coating by plasma arc spraying.

FIG. 5 is a sectional view taken along the line 5-5 of FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS ARC CI-IUTE STRUCTURE Referring now to FIG. 1 of the drawing, the circuit breaker shown therein comprises a pair of terminal bushings 1 and 2, both of which are fixed in position relative to the supporting frame of the circuit breaker. The bushing 2 comprises a downwardly extending conductive stud 3 at the lower end of which a movable conductive switch blade 4 is mounted by means of a fixed pivot 5. At its outer end, the blade 4 carries suitable cir cuit controlling contacts such as a main current carrying contact 6 and an arcing contact 7.

Bushing 1 comprises a conductive stud la to which a downwardly extending conductive member 8 is electrically connected. Attached to this conductive member 8 is a curved contact retaining member 9 which coacts with the member 8 to form a holding pocket for receiving the anchored ends of main stationary current carrying contact fingers 10. These fingers are pivotally mounted on a curved portion 12 of the conducting member 8 and are biased for limit rotative wiping movement in a closing direction by means of suitable compression springs 9a. These compression springs provide for high pressure circuit closing engagement between the stationary current carrying contact 10 and the movable main current carrying contact 6.

The movable arcing contact 7 cooperates with stationary arcing contact 13, which is mechanically and electrically connected to the conducting member 8 by suitable clamping means 14. The material of which the arcing contacts 7 and 13 are made is capable of with standing arcing and is also of relatively high resistivity in comparison to the material of the current carrying contacts 10 and 6. Accordingly, when the switch blade 4 is in its closed position as shown in FIG. 1 most of the circuit current flows through the current carrying contacts. It is only when the switch blade4 is driven counterclockwise to open the breaker that the arcing contacts carrying appreciable current. During contact opening action, the current carrying contacts part first, thereby diverting current through the arcing contacts which are still in engagement owing to their extensive wipe. Thereafter, the arcing contacts part and draw a circuit interrupting arc therebetween which is driven into an arc chute 15 and there lengthened, cooled and extinguished.

For driving the switch blade 4 counterclockwise to efiect circuit interruption, a reciprocable operating rod 16 pivotally joined to the switch blade at point 17 is provided. When this operating rod is driven upwardly, it acts to move the switch blade counterclockwise to effect a circuit interrupting operation. The circuit can be reestablished by driving the operating rod downwardly to return the switch blade 4 in a clockwise direction to the closed position illustrated in FIG. 1. The operating rod 16, which is made of insulating material, is actuated by means of a suitable conventional operating mechanism (not shown).

The are chute 15 comprises a pair of side walls 18 and 19 constructed of an arc-resistant and trackingresistant insulating material soon to be described in greater detail. As shown in FIG. 3, these side walls are clamped together in spaced apart relationship by means of insulating clamping strips and 21 and transverse bolts 20a extending between the strips on opposite sidewalls. Each side wall is provided with plurality of fins 22 projecting toward the other side wall and arranged to interleave with the corresponding projecting fins on the other side wall, thereby forming a sinous or zigzag passage as viewed either from the entrance end or from the exhaust end of the chute. The view from the latter end is illustrated in FIG. 3.

In the illustrated form of the invention, each side wall comprises a plurality of short ribs 11 which are respectively disposed between the fins 22 of the associated sidewall in spaced relation to the fins. These ribs 11 extend parallel to the fins 22 and are disposed in alignment with the fins on the opposite sidewall.

Referring to FIG. 2, the forward edges of the interleaving fins 22 taper toward the arc-initiation region at the right hand end of the chute. The tapering edges of each adjoining pair of fins 22 intersect as viewed in FIG. 2 to form an entrance angle such as a at the entrance to the zig-zag passage between the fins. The vertexes of these entrance angles all lie on a curvilinear line 23 (FIG. 1) which constitutes their locus. The region of the arc chute immediately adjacent this line 23 we refer to hereinafter as the vertex region of the arc chute.

For facilitating movement of the are into the arc chute, a pair of conductive arc runners 24 and 25 are provided along opposite edges of the chute. The sidewalls 18 and 19 extend between these runners 24 and 25. As shown in FIG. 1, runners 24 and 25 extend generally transverse to the axis of the arc and in divergent relationship with respect to each other from the arcinitiation region adjacent the separable contacts. The region of the arc chute where the diverging arc runners 24 and 25 are spaced apart by the least distance we refer to as the throat region of the arc chute. This throat region is designated 50 in FIG. 1.

In the throat region 50, there are two separate throat pieces 60, each constituting a sidewall with short fins, or ribs 62, integrally formed therewith. These throat pieces 60 are best shown in FIG. 5, where it can be seen that there is no interleaving of their ribs 62.

Projecting from and electrically connected to the upper runner 24 are two probes 51 of a refractory conductive material. The probes are located immediately adjacent but slightly spaced from the movable switchblade 4. The purpose of these probes is to accelerate transfer of the upper arc terminal to the upper arc runner 24.

The are chute further comprises a first group of blowout electromagnets 26, 27 and 28 mounted adjacent the upper arc runner 24 and a second group of blowout electromagnets 29, 30 and 31 mounted adjacent the lower arc runner 25. Each of these blow-out magnets comprises a generally U-shaped structure that comprises a pair of pole pieces 32 and 33 constituting the legs of the U and an interconnecting core member 34 between the legs of the U. Each blow-out magnet straddles the chute with its pole pieces mounted at the outer sides of sidewalls 24 and 25 and its core member extending between the sidewalls on the right hand side of the arc runners, as seen in FIG. 1. Mounted around the cores of the blow-out magnets are blow-out coils 26a, 27a, 28a, 29a, 30a and 31a. As shown, each blowout coil is round and its inside diameter is such that it snugly surrounds an insulating sleeve on the round cross section core on which it is mounted. The blowout coils in the upper group are connected in series relationship with each other, and those of the lower group are also connected in series with each other.

With respect to the electrical connection of the upper blow-out coils, note that the upper arc runner 24 is divided into a plurality of segments separated by insulating members 52 and 53. Blow-out coil 27a electrically bridges insulating member 52, and blow-out coil 28a electrically bridges insulating member 53. The first of the upper blow-out coils 26a is electrically connected between the stud 1a and the first segment of the arc runner. In a similar manner, the lower blow-out coils 30a and 31a electrically bridge insulating members 54 and 55 between segments of the lower runner. The first of the lower blow-out coils 29a is electrically connected between the first segment of the lower runner and the conductive stud 3 by means of an electric conductor 57.

When the contacts of the circuit breaker are closed, the blow-out coils are not in the power circuit and are consequently deenergized. When the contacts are separated to form an arc, movement of the are along the arc runners 24 and 25 into the chute connects these blowout coils in series with the arc in a known manner. An important purpose of the blow-out magnets is to propel the arc and accelerate its movement along the runners into the interior of the arc chute. When a particular blow-out coil is energized, the magnetic field produced between its pole pieces extends transversely of the arc column. This magnetic field reacts with the magnetic field surrounding the are in a known manner to produce a resultant force that drives the arc at high speed toward the interior of the chute.

In one form of the invention, the pole pieces 32 and 33 of the blow-out magnets are constructed in the manner disclosed and claimed in application Ser. No. 827,720-Frind et al., now US. Pat. No. 3,591,744 filed May 26, 1969, and assigned to the assignee of the present invention, and reference may be had thereto for a more complete description of the pole pieces and the manner in which they control the magnetic field that drives the are into the chute. In another form of the invention, which is the form used in the interrupting tests described hereinafter, all of the pole pieces 32 and 33 are of a highly laminated construction except for the pole pieces of electromagnet 29, which are nonlaminated or solid. 1

As the arc terminals move along the runners into the chute, the column of the arc moves freely into the chute until it encounters the intersecting forward edges of the interleaving fins 22 at the vertex region 23. Thereafter, further penetration of the arc chute causes the arc column to assume a zig-zag form as it bends around the overlapping edges of the fins 22, as shown in FIG. 2a. This elongation of the are and the cooling that results from the intimate engagement of the arc and fins are important factors contributing to successful interruption. It is to be noted that where the arc bends around the edge of a fin 22, it is positioned between the edge of the fin 22 and the edge of a rib 11 aligned with the fin and forming a part of the opposite side wall.

THE PLASMA-ARC SPRAYED COATING OF FUSED SILICA As was pointed out in the introductory portion of this specification, an object of ourinvention is to utilize in the arc chute insulating material that renders the arc chute capable of consistently interrupting substantially greater amounts of current than can an arc chute of corresponding size made of the widely used zirconfilled phospho-asbestos material referred to .hereinabove. We achieve this object, in a preferred form of our invention, by using for the sidewalls 18, 19 and fins 22 the same zircon-filled phospho-asbestos material as heretofore used; but instead of leaving the exposed surface of this material bare, we apply to a portion of this surface a plasma-arc sprayed coating of refractory metal-oxide consisting essentially of amorphous fused silica.

Amorphous fused silica is a non-crystalline material which, though identical in chemical composition to crystalline silica, has structural, thermal, and physical properties which are greatly different from those of crystalline silica. Structurally, the amorphous fused silica is a glassy substance substantially free of the crystals that constitute the crystalline forms of silica. From a thermal and physical standpoint, crystalline silica undergoes several transformations from one crystal form to another as it is heated to its melting point of about 1,700C. These transformations are accompanied by volume changes. If, on the other hand, amorphous fused silica is heated, no such transformations occur. At about l500C, the fused silica softens to a viscous liquid, which remains quite viscous even up to 200C. Amorphous silica and crystalline silica also have significantly different thermal expansion and thermal shock properties. When rapidly heated, crystalline silica, in passing through the above-described crystal transformations, with their associated volume changes, will be subjected to cracking and spalling; but amorphous fused silica, which passes through no such transformations, will be largely free from such cracking and spalling.

We apply our amorphous fused silica coating by relying upon a plasma-arc spraying process, schematically depicted in FIG. 4. Before the arc chute is assembled, each sidewall is placed in a substantially horizontal position, as shown in FIG. 4, so that its integrally formed fins 22 extend generally vertically. Then the upwardly facing surfaces 70, 71 and 72 of the component are coated by plasma arc spraying. Such coating is performed by utilizing a conventional plasma arc spray gun 74, such as disclosed for example in application Ser. No. 731,466-Bailey et al. now U.S. Pat. No. 3,588,433, filed May 23, 1968, and assigned to the assignee of the-present invention. As pointed out in that application, inside of the gun, a high current electric arc is formed and a suitable gas is passed through the region of the arc to form a stream of extremely hot arcplasma. The coating material, in this case, crystalline silica in the form of very pure quartz sand, is fed into the arc-plasma stream, where it is melted and converted into atomized particles of molten silica, which are ejected through a suitable nozzle at high velocity in the plasma stream. As shown in FIG. 4, the plasma stream containing the molten droplets is projected onto the upwardly facing surfaces -72 of the fins and the sidewalls. Upon striking the surfaces, the molten particles flatten and freeze into an adherent coating.

A microscopic examination of the coating shows that despite the fact that the source material fed into the plasma-arc spray gun is crystalline silica, the coating is almost entirely amorphous fused silica. Although a small amount of crystalline silica is present in the fused silica, our studies indicate that this amount is no more than a few percent, e.g., no more than about 5 per cent.

Microscopic examination of the coating shows further that the coating contains closed pores distributed throughout, with more closed pores near the substrate than near the outermost surface. Near the substrate, these pores seem to occupy about 25 to 35 per cent of the material volume and are larger than near the outer surface. The coating seems opaque near the substrate but more translucent near the outer surface. The coating has a glassy texture with a rather irregular surface, which becomes smoother as the arc chute is used for interruption.

The details of the arc-plasma spraying process are not considered to be a part of the present invention. For purposes of the present application, it is believed sufficient to point out that during the spraying process the gun 74 is moved transversely of the fins and that the source material used in the arc-plasma spraying process is quartz sand, a crystalline form of silica. The resultant coating, however, is amorphous fused silica, rather than the crystalline silica of the source material. This coating of amorphous silica has a high strength bond to the underlying substrate material. Efforts to apply the coating by using as a source material fused silica in powdered form resulted in the sprayed particles adhering poorly, if at all, to the substrate and in other spraying difficulties.

Our amorphous fused silica coating is highly resistant to cracking and spalling. As an example, we have repeatedly passed an oxy-acetylene flame across the coated surface of the arc chute without damaging the coating or the chute. In addition, examination of the coated arc chutes after high current interrupting tests typically showed no evidence of coating failure due to thermal shocks. The extreme high temperature of a high current are produces some erosion of the coating, but little or no spalling or break-off of the coating.

THE FUSED SILICA COATING OF EMBODIMENT In one form of the invention, referred to herein as Embodiment l, we apply a relatively thick coating of silica to the top surfaces 70 of the fins 22 depicted in FIG. 4 and to the top surfaces 71 of the ribs 11 on the sidewalls. By way of example, in one form of the invention, the thickness of the coating at 70 ranges from 13 to 20 mils and at 71 from 5 to 8 mils. Along the vertical sides of the fins of FIG. 4, the coating is only 1 or 2 mils in thickness and extends from the top surface about one-half the distance to the base of the fin. On the surfaces 72 the coating is a few mils in thickness. In the THE FUSED SILICA COATING OF EMBODIMENT II In another embodiment of our invention, referred to hereinafter as Embodiment II, excellent results have been obtained with an arc chute having fused silica coatings of a thickness only about one-half the thickness described hereinabove in the locations 70 and in the regions adjacent the runners 24 and 25 outside the throat region. Otherwise the coating was of about the same thickness as in Embodiment I. In making these latter are chutes, the base material was flame-treated before application of the coating by passing over its exposed surfaces the flame, or plasma, from an arc plasma torch. In the arc chute with the thicker coating of the preceding paragraph, no such flame-treatment was employed prior to applying the coating. Flame treatment before plasma-spraying serves to partially degas and partially remove absorbed and chemically bound water from the base material in the region adjacent its exposed surface, typically to a depth of about 6 to 10 mils. With the base material thus pretreated, it is unnecessary to prolong the subsequent plasmaspraying operation in order to develop the localized heat needed to accomplish sufficient degassing and water removal. Accordingly, the subsequent plasmaspraying operation on the pretreated base material can be performed relatively quickly, with a resultant thinner coating, and with much less heating of the arc chute. This reduced heating during spraying is advantageous in reducing warpage of the fins and sidewalls. The thinner coating also has the advantage of adding less to the flow impedance of the various passages through the arc chute.

BOTH EMBODIMENTS In both of the above embodiments, the phosphoasbestos arc chute components were baked in a conventional manner prior to spraying or flame-treatment.

In both embodiments, at the rear, or exhaust, end of the arc chute, there is a region 77 where the phosphoasbestos base material is left bare and uncoated. This is illustrated in FIG. 1, where the dotted vertical line 78 represents the boundary of the coated region. To the left of this line 78, the arc chute material is uncoated; but to the right of line 78, the arc chute is coated as described hereinabove. The reason for leaving this region 77 uncoated will soon be described. If the arc chute is subjected to the above-described flame pretreatment, the pretreatment covers the rear region 77 as well as the region to the right of line 78.

ADDITIONAL PROPERTIES OF THE FUSED SILICA COATING IN THE ARC CHUTE Amorphous fused silica has several important properties which contribute to its exceptional abilities as an arc chute material. One of these is that it evolves very little gas when exposed to high current arcs. Since it is so low in gas evolution, a high current arc can move along a surface of this material at a very high speed. There is little gas pressure built up ahead of the arc and thus little impedance to are motion. This low impedance is important in enabling the arc to move rapidly away from the arc-initiation region and deeply into the effective interrupting region behind the vertex 23. However, it is most important that the arc not move completely through the chute or that the arcing products not exhaust from the chute without adequate cooling, either of which conditions might cause the arc to restrike and burn across the rear of the chute, an intolerable failure mode. To prevent this from occurring, the rear portion 77 of the chute is left bare of coating, as above described, and its walls are therefore capable of evolving the usual relatively high quantity of gases. These gases serve to block are motion past the region of boundary line 78 and also to serve to efiectively cool the arcing products exhausted through the rear of the chute.

In the illustrated form of the invention, the surfaces of fins 22 that are disposed horizontally in FIG. 1 are only partially coated and even where coated, are coated with only a very light coating. This enables some gases to be evolved by the are from the underlying base material of the fins, and such gases are believed to provide a desirable cooling effect. The quantity of gases evolved from these regions is not so large, however, as to objectionably impede high speed motion of the are into the chute.

It is to be noted that the sidewalls of thearc chute in the region extending from the arc-initiation region to the region of vertex 23 are covered to a major extent with the fused silica. This is considered significant be cause the arc is located in this particular region when its current content is highest, and it therefore sees primarily fused silica during this high current interval. This effectively limits the amount of gas generated by the arc during the crucial high current interval, when gas-generation would otherwise be at its maximum.

Another important property of arc-plasma sprayed fused silica which contributes to its exceptional abilities as an arc chute material is that it is highly resistant to are erosion. The fused silica coating reduces erosion of the arc chute material to such an extent that the limit to the chutes life is imposed not by erosion but by metallic vapor deposits produced by the arcs vaporizing metal parts of the arc chute and contacts.

Still another important property of fused silica is that it has a very high electrical resistivity even when hot.

This strongly inhibits electrical current from flowing across'the chute immediately after arcing, thus further aiding in interruption.

INTERRUPTING TESTS WITH VARIOUS ARC CI-IUTES a. As an example of the improved interrupting ability of an arc chute utilizing the above-described fused silica coating, we were able in typical tests with the above-described Embodiment I of our arc chute (at 8.7 KV test voltage) to consistently interrupt current of 30,000 amperes r.m.s. total. In eight such high current tests, seven successful clearings of the circuit occurred.

b. With an arc chute of corresponding size, made of the same base material but with no coating, only one successful clearing occurred in two tests at the same current and voltage.

The uncoated arc chute of the previous paragraph had been subjected to the usual pretreatment before use that is conventionally relied upon for these chutes. This pretreatment involves baking the chute and later passing the flame from a plasma torch along its exposed surfaces. The coated chute of the previous paragraph was made without this flame-treatment step. The coating on the arc chute was the relatively thick coating of Embodiment I described hereinabove.

c. Interrupting tests on the arc chute of Embodiment II, described hereinabove, showed similar improved results. This are chute, it will be recalled, was made by first flame-treating the exposed surfaces of the base material with a plasma torch and then applying the relatively thin fused silica coating. In 18 interrupting tests with this chute (at 30,000 amperes r.m.s. total and 8.7 KV test voltage), 17 successful clearings occurred.

d. An arc chute made by the same process as in the immediately-preceding paragraph was tested at 13 KV and 20,000 amperes r.m.s. total. At this higher voltage,

14 interrupting tests at 20,000 amperes r.m.s. total resulted in 12 successful clearings.

e. Another arc chute was made by first flame-treating the exposed surfaces of the base material with a plasma torch and then plasma-arc spraying onto these surfaces a coating of 40 percent zinc oxide, 60 percent silica. In three interrupting tests on this chute at 13 RV and 20,000 amperes r.m.s. total, only one successful clearing occurred.

f. Still another arc chute was made by first flametreating with a plasma torch and then plasma-arc spraying onto its exposed surface zirconium oxide. In one interrupting test on this chute at 13 KV and 20,000 amperes r.m.s. total, no successful clearing occurred.

g. Still another arc chute was made by first flametreating with a plasma torch and then plasma-arc spraying onto its exposed surface zirconium silicate. In two interrupting tests on this chute at 13 KV and 20,000 amperes r.m.s. total, no successful clearings occurred.

h. Still another arc chute was made by first flametreating with a plasma torch and then plasma-arc spraying onto its exposed surface mullite (3Al O .2 SiO,). In one interrupting test at 13 KV and 20,000 amperes r.m.s. total, no successful clearings occurred.

All of the arc chutes used in the tests described hereinabove were made of the same zircon-filled phosphoasbestos base material.

These interrupting tests may be tabulated as follows:

(h) Coating of 3 Al,O,-2 SiO,

In this table the letters appearing in the left hand column in parentheses correspond to letters appearing hereinabove in the text, and reference may be had to such letters in the text for a more detailed description of the particular chute involved. Under Results, the first number indicates the number of interrupting tests and the second number indicates the number of successful clearings in these tests.

It will be apparent from these test results that the disclosed arc chute performed high current interruptions considerably more consistently with the fused silica coating than with any of the other coatings referred to in the test results. It will also be apparent that high current interruptions were performed considerably more consistently with the fused silica coated arc chute than with the uncoated arc chute.

It is to be understood that the above interrupting tests were single phase tests run directly from a short circuit generator. It is to be further understood that these tests were conducted with suitable back-up interrupting means available for interrupting the current through the tested circuit breaker in the event that the tested breaker failed to clear the circuit. This back-up means can operate with sufficient rapidity to prevent the tested breaker from being damaged in the event that it fails to clear.

It is to be further understood that in the arc chutes used in the above tests, there was a muffler for cooling exhaust gases disposed at the exhaust end of the arc chute. A muffler of the general type used is shown in U.S. Pat. No. 3,555,224-Frind et al., assigned to the assignee of the present invention.

ADDITIONAL DISCUSSION We have considered constructing the sidewalls and fins in their entirety from fused silica instead of merely with a fused silica coating. Our studies indicate that this approach is of questionable practicality in a large arc chute because large parts of intricate shape constructed of fused silica are quite brittle and fragile. Furthermore, such material does not lend itself to machining. Our phospho-asbestos material, on the other hand, is much stronger mechanically and can be easily molded and machined.

In a typical arc chute embodying our invention, the height h in the region of maximum height, as seen in FIG. 1, was about 31 inches; the length l in the region of maximum length was about 13% inches; the sidewalls were spaced apart by a distance w (FIG. 2) of about 1% inches; the fins 22 and the ribs 11 were each about five thirty-seconds inches in thickness (as seen in FIG. 2a); and the spacing s (FIG. 2a between adjacent fins and ribs was about 0.1 inch. Arc chutes of approximately these dimensions were used in the abovedescribed interrupting tests. These dimensions are given by way of example and not limitation.

Although our invention in its broader aspects is not intended to be limited to a phospho-asbestos base material, it should be noted that this material is especially suitable as a base material because it is refractory and can be heated to red heat without damage. This is important because the arc-plasma spraying process does heat the fins to red heat while they are receiving their fused silica coating.

While we have shown and described particular embodiments of our invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from our invention in its broader aspects; and we, therefore, intend herein to cover all such changes and modifications as fall within the true spirit and scope of our invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An arc chute for an alternating current electric circuit breaker into which an arc is adapted to be driven for the purpose of extinguishing the arc comprising:

a. means defining an arc-initiation region in which the arc is initiated during an interrupting operation,

b. a pair of conductive arc runners spaced apart within said chute for providing paths along which the terminals of said are travel as the arc is driven into said chute from said arc-initiation region,

c. a pair of spaced-apart sidewalls of insulating material extending generally parallel to the path followed by said are as it moves into said chute,

d. spaced-apart plates of insulating material extending transversely of said sidewalls and having edges for engaging said arc as it moves into said chute,

e. said edges defining a vertex region beyond which further motion of the are into the chute causes the arc to develop a zig-zag configuration looping around the edges of said plates,

d. said edges and portions of said sidewalls aligned therewith being covered with an adherent coating of plasma-arc-sprayed refractory metal-oxide consisting essentially of amorphous fused silica.

2. An arc chute for an alternating current electric circuit breaker into which an arc is adapted to be driven for the purpose of extinguishing the arc, comprising:

a. means defining an arc-initiation region in which the arc is initiated during an interrupting operation, b. a pair of spaced-apart sidewalls of insulating material extending generally parallel to the path followed by said arc as it moves into said chute from said arc-initiation region,

c. spaced-apart plates of insulating material extending transversely of said sidewalls and having edges for engaging said arc as it moves into said chute,

d. said edges being covered with an adherent coating of plasma-arc-sprayed refractory metal-oxide consisting essentially of amorphous fused silica.

3. The are chute of claim 2 in which both of said sidewalls have fins projecting therefrom and constituting said plates, the fins on one sidewall interleaving with the fins on the other sidewall and having their edges spaced from said other sidewall, each of said sidewalls having portions aligned with the fins-of the other sidewall, said portions also being covered with an adherent coating of plasma-arc-sprayed refractory metal-oxide consisting essentially of amorphous fused silica.

4. The arc chute of claim 2 in which said sidewalls in the region of said are chute bordering said arcinitiation region are covered with an adherent coating of plasma-arc-sprayed refractory metal-oxide consisting essentially of amorphous fused silica.

5. The arc chute of claim 2 in which said sidewalls and said plates are of zircon-filled phospho-asbestos material to which said coating is bonded.

6. The are chute of claim 2 in which said sidewalls and said plates are of phospho-asbestos material to which said coating is bonded.

7. The arc chute of claim 2 in which said plates have surfaces extending transversely of said sidewalls, said surfaces having portions adjacent said sidewalls which are substantially free of plasma-arc-sprayed coating.

8. The are chute of claim 7 in which said plates are of phospho-asbestos material.

9. The arc chute of claim 2 in which said plates are .of a material that is much more gas-evolving than said fused silica coating.

10. The are chute of claim 2 in which said edges define a vertex region beyond which further motion of the are into the chute causes the arc to develop a zig-zag configuration looping around the coated edges of said plates.

11. The are chute of claim 10 in which the sidewalls of said are chute in the region extending from said arcinitiation region to said vertex region are covered with an adherent coating of plasma-arc-sprayed refractory metal-oxide consisting essentially of amorphous fused silica.

12. The are chute of claim 2 in which:

a. said arc chute has an exhaust portion remote from said are initiation region through which arcing products are exhausted from said chute,

b. the surfaces of said exhaust portion are generally free of said fused silica coating and of a much more gas-evolving material than said fused silica coating.

13. The arc chute of claim 1 in which the arc chute has a maximum dimension measured transversely of said plates of greater than 20 inches and a maximum dimension measured along the length of said plates of greater than 8 inches.

14. The are chute of claim 2 in which the arc chute has a maximum dimension measured transversely of said plates of greater than 20 inches and a maximum dimension measured along the length of said plates of greater than 8, inches.

15. The are chute of claim 2 in which said sidewalls and said plates are of an asbestos-type base material from which chemically bound water has been partially removed over a region extending at least several mils in depth from the surface of said are chute that is exposed to said arc.

16. The arc chute of claim 3 in which:

a. the fins on one of said sidewalls have surfaces extending transversely of said one sidewall, which surfaces have portions adjacent said one sidewall that are substantially free of plasma-arc-sprayed coating, and

b. the fins on the other of said sidewalls have surfaces extending transversely of said other sidewall, which surfaces have portions adjacent said other sidewall that are substantially free of plasma-arc-sprayed coating.

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2. An arc chute for an alternating current electric circuit breaker into which an arc is adapted to be driven for the purpose of extinguishing the arc, comprising: a. means defining an arc-initiation region in which the arc is initiated during an interrupting operation, b. a pair of spaced-apart sidewalls of insulating material extending generally parallel to the path followed by said arc as it moves into said chute from said arc-initiation region, c. spaced-apart plates of insulating material extending transversely of said sidewalls and having edges for engaging said arc as it moves into said chute, d. said edges being covered with an adherent coating of plasma-arc-sprayed refractory metal-oxide consisting essentially of amorphous fused silica.
 3. The arc chute of claim 2 in which both of said sidewalls have fins projecting therefrom and constituting said plates, the fins on one sidewall interleaving with the fins on the other sidewall and having their edges spaced from said other sidewall, each of said sidewalls having portions aligned with the fins of the other sidewall, said portions also being covered with an adherent coating of plasma-arc-sprayed refractory metal-oxide consisting essentially of amorphous fused silica.
 4. The arc chute of claim 2 in which said sidewalls in the region of said arc chute bordering said arc-initiation region are covered with an adherent coating of plasma-arc-sprayed refractory metal-oxide consisting essentially of amorphous fused silica.
 5. The arc chute of claim 2 in which said sidewalls and said plates are of zircon-filled phospho-asbestos material to which said coating is bonded.
 6. The arc chute of claim 2 in which said sidewalls and said plates are of phospho-asbestos material to which said coating is bonded.
 7. The arc chute of claim 2 in which said plates have surfaces extending transversely of said sidewalls, said surfaces having portions adjacent said sidewalls which are substantially free of plasma-arc-sprayed coating.
 8. The arc chute of claim 7 in which said plates are of phospho-asbestos material.
 9. The arc chute of claim 2 in which said plates are of a material that is much more gas-evolving than said fused silica coating.
 10. The arc chute of claim 2 in which said edges define a vertex region beyond which further motion of the arc into the chute causes the arc to develop a zig-zag configuration looping around the coated edges of said plates.
 11. The arc chute of claim 10 in which the sidewalls of said arc chute in the region extending from said arc-initiation region to said vertex region are covered with an adherent coating of plasma-arc-sprayed refractory metal-oxide consisting essentially of amorphous fused silica.
 12. The arc chute of claim 2 in which: a. said arc chute has an exhaust portion remote from said arc initiation region through which arcing products are exhausted from said chute, b. the surfaces of said exhaust portion are generally free of said fused silica coating and of a much more gas-evolving material than said fused silica coating.
 13. The arc chute of claim 1 in which the arc chute has a maximum dimension measured transversely of said plates of greater than 20 inches and a maximum dimension measured along the length of said plates of greater than 8 inches.
 14. The arc chute of claim 2 in which the arc chute has a maximum dimension measured transversely of said plates of greater than 20 inches and a maximum dimension measured along the length of said plates of greater than 8 inches.
 15. The arc chute of claim 2 in which said sidewalls and said plates are of an asbestos-type base material from which chemicalLy bound water has been partially removed over a region extending at least several mils in depth from the surface of said arc chute that is exposed to said arc.
 16. The arc chute of claim 3 in which: a. the fins on one of said sidewalls have surfaces extending transversely of said one sidewall, which surfaces have portions adjacent said one sidewall that are substantially free of plasma-arc-sprayed coating, and b. the fins on the other of said sidewalls have surfaces extending transversely of said other sidewall, which surfaces have portions adjacent said other sidewall that are substantially free of plasma-arc-sprayed coating. 